In recent years, photonic crystals created with fine particles have attracted considerable interest (Non-patent Document 1) because the light emission and transmittance thereof can be artificially controlled with fine particles. Fine particles for use in photonic crystals require the following properties: they must be spherical, have a particle size of about 50 nm to 200 nm, have a narrow particle size distribution (small standard deviation in particle size), have a high refractive index (n>2), and have good dispersibility in liquids. However, fine particles satisfying these conditions have not been developed until now.
Meanwhile, cerium oxide has a high refractive index of 2.1 and is a suitable material for photonic crystals (Non-patent Documents 2 and 3). Moreover, cerium oxide is well known for its UV light-blocking effects and, for example, a UV light-blocking agent using cerium oxide is disclosed in a prior art document (Patent Document 1). Textiles, etc., with UV light-blocking effects are needed because UV light is harmful to the human body, and cerium oxide shows promise for use in those kinds of textiles.
Spherical CSCP nanoparticles have recently been reported in prior art (Patent Document 2, Non-patent Document 4). The spherical CSCP nanoparticles in these reports are unlike others (Non-patent Documents 5 to 8) and have the following characteristics: (1) the shape of these nanoparticles is spherical, (2) the particle size distribution of the nanoparticles is narrow, (3) the nanoparticles comprise cerium oxide primary particles of 2 nm to 3 nm aggregated in spherical to form core-shell structures coated with an organic polymer, (4) even if the nanoparticles are dried, they can be very easily re-dispersed in an aqueous or nonaqueous solvent, (5) the nanoparticles can be dispersed in a liquid with a high particle concentration to prepare a dispersion thereof, (6) the mean particle size of the nanoparticles can be controlled while the particle size distribution remains narrow, e.g. within a range of 50 nm to 120 nm, by changing the molecular weight of the polymer that is added during fabrication, and (7) the particles can be synthesized easily with the polyol method.
These spherical CSCP nanoparticles show promise as colloidal crystals because they are spherical and have a narrow particle size distribution (as monodispersion). Moreover, it is expected that these spherical CSCP nanoparticles can be dispersed in a variety of resins because they are very easily re-dispersed in an aqueous or nonaqueous solvent. Furthermore, an aggregate of spherical CSCP nanoparticles is disclosed in one of the above prior art documents (Patent Document 2).
There is a problem, however, because the aggregate disclosed in the above prior art document (Patent Document 2) has no mechanical strength and is so friable that it will break if grasped with tweezers. Moreover, in that document the self-organized, nonuniform assemblages without any fixing agent of the nanoparticles are referred to as aggregates. Therefore, a film with a large surface area that uniformly contains spherical CSCP nanoparticles could not be produced with the technology disclosed in that document.
Furthermore, one prior art document (Patent Document 3) discloses a composite wherein a polymer is covalently bonded to the surface of fine inorganic oxide primary particles and a coating composition (resin) containing the same, but those are fundamentally different from spherical CSCP nanoparticles and an aggregate thereof. Based on their explanation, the fine inorganic oxide particles disclosed in Patent Document 3 are understood to be primary particles. In contrast, spherical CSCP nanoparticles comprise spherical secondary particles into which primary particles gathered, so the two are entirely different. Moreover, because the shape of the fine inorganic oxide particles in Patent Document 3 is described as random, the shapes of the two differs as well.
In general, it is rare for fine inorganic oxide primary particles to be spherical. This is because primary particles are crystals, so crystal planes are often present on the surfaces thereof. Primary particles of a random shape often agglomerate, and secondary particles formed therefrom have a shape similar to a cluster of grapes. Therefore, because a polymer is covalently bonded to the surfaces of the grape cluster-shaped secondary particles in Patent Document 3, each particle will have a different shape. As a result, such secondary particles cannot be dispersed at a high density in a resin.
Quite recently the inventors attempted to fix the above spherical CSCP nanoparticles in a photosetting resin (photosensitive resin), but they encountered a problem because the spherical CSCP nanoparticles aggregated in self-organized, nonuniform agglomerations, and parts that only contained resin appeared, so the inventors were not able to obtain a uniform aggregate. Moreover, when ethanol, propylene glycol monomethyl ether, or ethylene glycol monobutyl ether was used as the solvent, problems with light transmittance, nonuniformity, etc., occurred. Therefore, there is a strong demand in this field for the development of a spherical CSCP nanoparticle aggregate having a uniform distribution of spherical CSCP nanoparticles and excellent mechanical strength, the technology for producing the same, and products using the same.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-35632    Patent Document 2: Japanese Patent Application Laid-open No. 2008-115370    Patent Document 3: Japanese Patent Application Laid-open No. 2003-041152    Non-patent Document 1: Shuichi Shibata, Ceramics 41 (2006) 334    Non-patent Document 2: M. G. Krishna, A. Hartridge, A. K. Bhattacharya, Materials Science and Engineering B55 (1998) 14    Non-patent Document 3: M. Mogensen, N. M. Sammes, G. A. Tompsett, Solid State Ionics 129 (2000) 63    Non-patent Document 4: N. Izu, I. Matsubara, T. Itoh, W. Shin, M. Nishibori, Bulletin of the Chemical Society of Japan 81 (2008) 761-766    Non-patent Document 5: C. Ho, J. C. Yu, T. Kwong, A. C. Mak, S. Lai, Chem. Mater., 17 (2005) 4514    Non-patent Document 6: N. Uekawa, M. Ueta, Y. J. Wu, K. Kakegawa, J. Mater. Res., 19 (2004) 1087    Non-patent Document 7: X. Chu, W. Chung, L. D. Scmidt, J. Am. Ceram. Soc., 76 (1993) 2115    Non-patent Document 8: W. P. Hsu, L. Ronnquist, E. Matijevic, Langmuir 4 (1988) 31
Given these circumstances, while giving due consideration to the prior art, the inventors performed intensive research with the aim of producing a new aggregate of spherical CSCP nanoparticles that comprises an aggregate body formed by uniformly distributed, aggregated CSCP nanoparticles synthesized by the polyol method, that has excellent mechanical strength, that can increase surface area, and that has excellent transparency. As a result, by using a special solvent and fixing agent the inventors succeeded in producing a spherical CSCP nanoparticle aggregate that is highly concentrated with the volume fraction of spherical CSCP nanoparticles being 32% or more, that has a uniform distribution of spherical CSCP nanoparticles, and that has excellent mechanical strength, thus completing the present invention. An object of the present invention is to provide an aggregate of spherical CSCP nanoparticles that has excellent mechanical strength, that can increase surface area, and that can form a transparent film at low cost; and a process for producing the same. A further object of the present invention is to provide an anti-reflection material containing a high refractive index layer formed by using such an aggregate of spherical CSCP nanoparticles.
The present invention comprises the following technical means for solving the above problems.
(1) An aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles,
the aggragate having uniformly distributed and aggregated spherical core-shell cerium oxide/polymer hybrid nanoparticles having a spherical secondary particle formed by assembled cerium oxide primary particles serving as the core and a polymer serving as the shell, wherein 1) the aggregate is highly concentrated, with a volume fraction of the spherical core-shell cerium oxide/polymer hybrid nanoparticles being at least 32%, 2) the aggregate contains a fixing agent made from a resin, 3) the aggregate is configured such that the fixing agent fills in voids between the spherical core-shell cerium oxide/polymer hybrid nanoparticles, 4) the aggregate has a uniform distribution of the spherical core-shell cerium oxide/polymer hybrid nanoparticles, and 5) the aggregate has mechanical strength.
(2) The aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles according to (1) above, wherein the above fixing agent is a photosetting resin or a thermosetting resin that has been cured by irradiation with light or application of heat.
(3) The aggregate of spherical core-shell oxide/polymer hybrid nanoparticles according to (1) above, wherein no silane coupling agent or surfactant is present on a surface of the core of the spherical core-shell cerium oxide/polymer hybrid nanoparticles.
(4) The aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles according to (1) above, wherein the aggregate has a film-like structure formed as a coating on a substrate or base material.
(5) The aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles according to (1) above, wherein a refractive index thereof is no lower than 1.65.
(6) The aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles according to (1) above, wherein a haze thereof is no higher than 2%.
(7) An anti-reflection film characterized by comprising an aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles defined in (1) above.
(8) An ink for fabricating an aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles defined in (1) above, wherein the ink is formed from spherical core-shell cerium oxide/polymer hybrid nanoparticles, a photosetting resin, and a solvent.
(9) A process for producing an aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles defined in (1) above, wherein the process comprising: a step of preparing an ink formed from core-shell cerium oxide/polymer hybrid nanoparticles, a photosetting resin or a thermosetting resin, and a solvent; a step of forming the ink into a compact; and a step of curing the resulting compact by irradiation with light or application of heat.
(10) The process for producing the aggregate of spherical core-shell cerium oxide/polymer hybrid nanoparticles according to (9) above, wherein the solvent is methyl ethyl ketone, methyl isobutyl ketone, methyl lactate, ethyl lactate, or butyl lactate.